ORIGINAL: BobCo19-65
What I like to do is simply take the string that is on the bow at correct brace heightand measure it (well, you have to unstring it first). Put one end of the loop on a nail or hook, stretch it out, mearure it inside loop to inside loop, then give the measurement to Chad to build me one.
That's how I'd do it, if I didn't know the AMO length. With your current string, adjust the bow to theproper brace height. Take the string off the bow, stretch it between two nails and measure. If you want to actually measure the bow for AMO length, here's a good article that explains things pretty well concerning AMO bow lengths and proper string lengths for traditional equipment.
http://www.bowhunting.net/artman/publish/TailorMaidMeasuring.shtml